Right now, I use Usenetserver, with 35 day retentions, but 10 connections. PowerUsenet has the great 50 day retention rate, but only 4 connections.

How does the number of connections differ? Right now I have a 15mbps cable connection, I can max it out with usenetserver, will the fewer connections of Powerusenet cause my downloads to be slower? how does the numbeof connections effect my service?

It all depends on what speed you can get over individual connections. The general rule of thumb is to use the smallest number of connections that can max out your bandwidth. Once you can max out your bandwidth then using extra connections is pointless, and can even be slightly counter-productive due to the increased overhead of handling the extra connections.